The most effective way to use less fuel is to accelerate more slowly and avoid braking hard whenever possible. People driving trucks might also want to remove as much as they can from the truck bed.
Engines are most efficient when they are within a small RPM band, and accelerating slowly allows the transmission to keep the engine in this range as much as possible. Braking converts forward momentum to heat, which reduces the vehicle's mileage. Keeping the tires inflated properly can lead to slightly better fuel efficiency, and making sure the air filter is replaced at regular intervals can save on gas money. Having the tires realigned can save fuel and prevent premature damage to the tires.